DE 196 32 977 A describes a field chopper of the initially cited type in which the rotational speeds of the chopper drum and of the draw-in or feed rollers are detected by sensors. The quotient of the rotational speeds, that is a measure for the cut length of the chopped crop, is compared with a given desired value. In case of a deviation from the desired value, the rotational speed of the draw-in rollers and/or of the chopper drum is varied in such a manner that the difference between the desired and the actual rotational speed ratio is at least reduced. To this end, the chopper drum and/or the draw-in rollers are hydraulically driven. The speed of these elements is therefore regulated, which can entail disadvantageous fluctuations of control. Moreover, a sensor for the rotational speed of the draw-in rollers is necessary that must be arranged at a position at which it is exposed to the influence of vibrations and plant juices. It thus has a relatively short service life unless resort is made to very expensive embodiments.